CORRELATION OF THE CENOZOIC 203 



Putting together all these facts of various kinds, the first result 

 is the proof that the mammalian life of Eurasia, and America in 

 Tertiary times passed through a series of grand phases of union, 

 of divergence, of reunion, and perhaps again of divergence. There 

 are seven of these phases. 



In the prst, in Basal Eocene times, we find North America, Europe, 

 and possibly South America peopled with archaic mammals of 

 Mesozoic ancestry. 



In the second faunal phase, of Lower Eocene times, we observe the 

 first modernization, which occurs simultaneously in Europe and 

 North America, by the invasion of many modern families of mammals, 

 which intermingled with the archaic; the life of Europe and North 

 America continues to be very similar. 



In the third faunal phase, beginning in Middle Eocene times, the 

 mammals of America and Europe gradually diverge and undergo an 

 independent evolution with little or no faunal interchange; at the 

 close of the Eocene the two faunas are very far apart. 



In the fourth faunal phase, beginning in Lower Oligocene times, 

 there is a sudden reunion of New and Old World life. At the same 

 time there occurs in both countries a second very surprising moderni- 

 zation apparently by the further invasion of modern forms from the 

 north. 



A fifth faunal phase occurs in the Middle Miocene, when there is 

 a fresh reunion in the New and Old Worlds by the arrival in America 

 of the proboscideans and the short-limbed rhinoceroses. 



Then follows a long period of independent evolution in the two 

 countries until in the Middle Pliocene we enter a sixth faunal phase, 

 in which a close land connection with South America is re-established, 

 after an interval of separation reaching back into Eocene times. 



Finally a seventh faunal phase occurs in Pleistocene or Glacial 

 times, when all the larger North American mammals become extinct, 

 as well as the south American invading stocks, while North America 

 is replenished by a large fauna from Eurasia. 



It will be noticed that these phases are in no way coincident either 

 with the greater or with the lesser time divisions, for the obvious 

 reason that these time divisions have all been established on the 

 basis of the evolution of invertebrate life in Europe. 



